An optical fiber coupler couples light into and out of an optical fiber and is an indispensable part of any optical communication system. However, coupling light into and out of an optical fiber presents a number of problems. In the case of multimode fibers, a lens must be positioned and configured to focus the light of a well-collimated beam to a spot size that is less than the diameter of the fiber core. In addition, the angle from the lens to the fiber must be less than the numerical aperture (“NA”) of the fiber core. In others words, the position and configuration of the lens used to inject light into the core of an optical fiber are selected to ensure that the maximal ray of the focused beam lies within the NA of the fiber core, in this way effective coupling can be accomplished. On the other hand, in order to couple light into a single mode fiber, the mode of the fiber core and the mode of the light must be nearly phase matched. For example, in order to couple light of a wavelength λ from a collimated Gaussian laser beam of 1/e2 and diameter D into a single-mode fiber with a mode-field diameter d, a lens with a focal length give by:∫=D(πd/4λ)is used. Coupling efficiency depends on the overlap integral of the Gaussian mode of the input laser beam and the nearly Gaussian fundamental mode of the fiber.
However, the lenses typically used to couple light into and out of either single or multimode fibers require a relatively large optical fiber coupler to house and position the lens at a fixed distance from the end of the fiber. In addition, the lens is typically the most expensive component of the optical fiber coupler.
Physicists and engineers continue to seek enhancements that reduce the cost and size of optical fiber couplers.